.

It appears to me that, just as among the existing _Carnivora_, the
walruses and the eared seals are intercalary forms between the fissipede
Carnivora and the ordinary seals, so the Zeuglodonts are intercalary
between the _Carnivora_, as a whole, and the _Cetacea_. Whether the
Zeuglodonts are also linear types in their relation to these two groups
cannot be ascertained, until we have more definite knowledge than we
possess at present, respecting the relations in time of the _Carnivora_
and _Cetacea_.

Thus far we have been concerned with the intercalary types which occupy
the intervals between Families or Orders of the same class; but the
investigations which have been carried on by Professor Gegenbaur,
Professor Cope, and myself into the structure and relations of the
extinct reptilian forms of the _Ornithoscelida_ (or _Dinosauria_ and
_Compsognatha_) have brought to light the existence of intercalary forms
between what have hitherto been always regarded as very distinct classes
of the vertebrate sub-kingdom, namely _Reptilia_ and _Aves_. Whatever
inferences may, or may not, be drawn from the fact, it is now an
established truth that, in many of these _Ornithoscelida_, the hind limbs
and the pelvis are much more similar to those of Birds than they are to
those of Reptiles, and that these Bird-reptiles, or Reptile-birds, were
more or less completely bipedal.

When I addressed you in 1862, I should have been bold indeed had I
suggested that palaeontology would before long show us the possibility of
a direct transition from the type of the lizard to that of the ostrich.
At the present moment, we have, in the _Ornithoscelida_, the intercalary
type, which proves that transition to be something more than a
possibility; but it is very doubtful whether any of the genera of
_Ornithoscelida_ with which we are at present acquainted are the actual
linear types by which the transition from the lizard to the bird was
effected. These, very probably, are still hidden from us in the older
formations.

Let us now endeavour to find some cases of true linear types, or forms
which are intermediate between others because they stand in a direct
genetic relation to them. It is no easy matter to find clear and
unmistakable evidence of filiation among fossil animals; for, in order
that such evidence should be quite satisfactory, it is necessary that we
should be acquainted with all the most important features of the
organisation of the a